Fiber optic sensors can be used for many sensing applications. This includes automotive, medical security and industrial processing. Fiber optic interferometric sensors can provide the highest performance of any fiber optic sensor, however, fiber optic interferometric sensors require electronic processing to convert the optical information inscribed in the sensor to a linear electrical output. Thus, electronic circuitry is associated with fiber optic interferometric sensors. Fiber optic sensors are considerably more attractive for sensing applications when the sensor can be remotely deployed with the electronic processing and the reference transducer separated and removed from the sensor. One approach has been coherence multiplexing of fiber optic interferometric sensors with highly coherent sources as described by Brooks et al., "Coherent Multiplexing of Fiber-Optic Interferometric Sensors." IEEE Journal of Lightwave Technology, Vol. LT-3, No. 5 at page 1062, (October 1985). Large coherence lengths (4.5 m), however, are not compatible with small sensor designs. Moreover, these highly coherent sources are typically single mode or quasi-single mode laser diodes.
This specification will describe a system which combines these desirable features:
a fiber optic interferometric sensor system which utilizes short or very small coherence lengths (on the order of 10 microns to 10 centimeters) where the reference transducer and the electronic processing circuits are located remotely from the sensor.
A Coherence Selective Fiber Optic Interferometric Sensor System has numerous advantages as described in U.S. Pat. No. 5,094,534. That patent describes the utility of coherence selective sensors using an acoustic sensor as an example. The present disclosure provides a detailed description of an electric field sensor/converter for use in antenna remoting applications and also provides several improvements to the '534 patent. The improvements include the use of a sensor or transducer which requires a preferred polarization orientation, the use of a transmissive sensor and the use of a transmissive reference transducer. Also described is a transmitter-receiver configuration incorporated into the Coherence Selective Fiber Optic Interferometric Sensor System.
The present invention describes, in detail, specific system implementations and applications for a coherence selective fiber optic interferometric sensor system which provide for employing optical sensors for communications applications, lower optical loss through the use of transmissive sensors and reference transducers, and the use of polarization dependent sensors. A Passive Antenna Remoting System (PARS) employing a coherence selective electric field sensor/electric field converter is used as an example of these system implementations.
Currently deployed conventional microwave communication antennas are usually located in rugged terrain on mountains where services such as air conditioning as required for electronic processing equipment are at a premium. The high loss of coaxial cable often means that power must be provided along the transmission line to power amplifiers. Maintenance is also required at the antenna site and along the transmission line. These disadvantages are so great that microwave transmissions are often down converted to intermediate frequencies before transmission. Additional equipment is required at the antenna site in order to down convert. Therefore, it is desirable to use a low loss fiber optic transmission line to achieve long transmission distances without loss of signal or frequency down conversion. Currently fiber optic remoting of microwave antennas is accomplished by connecting (via an optical fiber) a fiber optic transmitter at a remote antenna site with a receiver located at a communication analysis site, such as described in U.S. Pat. No. 5,042,086. In a sensor-based antenna remoting system as discussed in this disclosure, a fiber optic transmitter at the antenna site is replaced by a passive fiber optic sensor. Ideally, no electrical power is required at the antenna/sensor location. In practice, however, some power is required to operate microwave or RF preamplifiers. No power is required at the antenna to power any optical component.
The coherent selective sensor disclosed has several advantages over conventional fiber optic transmission systems. Fiber optic transmitters require optical sources such as lasers. Lasers function best at lower temperatures and are often incompatible with the operational environment at the antenna. Like conventional coaxial cable transmission, an environmentally controlled shed at the antenna site would be required. In the coherent selective sensor system disclosed herein, an integrated optics modulator inscribes the information received by the antenna directly onto the light passing through the modulator. No electrical power other than the electrical signal to be transmitted is required by the modulator. The light source for the modulator is collocated with the optical receiver at an information processing center. Since personnel operate the processing centers, the environmental conditions are benign in comparison to those at the antenna site. Thus, the disclosed system may have may applications where a standard fiber optic transmission system would not be possible.
Integrated optic modulators are considered to be temperature sensitive. The operating point of the modulators do in fact vary with temperature. In the disclosed configuration where the modulator is located at the antenna, any change in the modulator operating point is compensated at the information processing center with the reference transducer.
Conventional fiber optic transmission systems whether using direct modulation of the laser source or external modulation, amplitude modulate light and pass that amplitude information over the optical fiber. Amplitude in this sense includes 1 and 0 which allows digital transmission. The coherence selective sensor systems disclosed herein offer the additional advantage that the information is phase modulated and the two incoherent signals with the relative phase modulation inscribed are transmitted over the same fiber. Due to the incoherence of the signal and the phase modulation, no information is encoded as amplitude variations, thus providing a level of security. A reference transducer which matches the time delay generated between the two signals at the modulator is required to recover the information. The transmission of both incoherent signals over the same transmission line results in common mode cancellation of any noise or perturbation applied to the fiber transmission line. This includes common mode cancellation of fiber dispersion.
The common mode cancellation of fiber dispersion is extremely important, in that singlemode lasers are typically used for long transmission distances, to over come dispersion. Singlemode lasers suffer from the fact that Stimulated Brillouin Scattering (SBS) limits the maximum optical power which a fiber can transmit and that the onset of SBS is lower as the linewidth is narrowed. The common mode cancellation of dispersion allows the use of lower cost multimode, broadband of multi-line lasers for long transmission distances. Significantly more optical power can be launched into the fiber, and therefore longer transmission distances without a repeater can be achieved. Even multimode fibers of standard or specialized designs may be used for high frequency information since the larger dispersion experienced with multimode fibers is common to both signals. The use of multimode fibers also allows for greater optical powers to be transmitted since the area of the optical fiber over which the light propagates is larger.
The following is an example of one possible application of the disclosed system; a new microwave antenna is to be installed 10 kilometers out of town. A microwave low noise amplifier and an electric field converter is connected to the antenna and mounted directly exposed to the environment without the necessity of building an antenna shed. Other than the power needed to drive the RF preamplifier (which due to the low power requirements can be driven remotely over small conductor included in the fiber cable), no additional power is required. Both the transmitter and receiver are located at the information processing center, such as a Cable TV Headend. No repeaters, down converters or other equipment is required between the antenna and the processing center. All high value and repairable equipment is located in the center where operators function on a daily basis. If transmitted signals are analog in nature, dynamic ranges of between 140 and 160 dB in a 1 Hertz bandwidth can be maintained. The upper limit will improve as new components become available.
A second application of the disclosed system is a replacement for conventional transmitter receiver systems. This configuration is called STARS for Secure Transmit and Receive System. Many applications, such as building-to-building transmission, do not have environment requirements of antenna remoting. In this case, the security and common mode rejection advantages of the disclosed system can be maintained, but at lower cost. The integrated optics modulator is packaged with the laser source at the transmitter and other support electronics. Two optical signals are generated and propagated over a single fiber to a matching reference transducer and a receiver at the other end of the transmission line. Similar performance to that described above is available using this configuration.
Another application of the coherence selective sensor system is for vibration sensing. An optical or fiber optic device which senses optical path length induced by vibration can be mounted on power generators. The Electro Magnetic Interference in this environment limits the utility of conventional sensors for this application. The optical source, the receiver, and the associated electronics would all be located in a monitor and control facility which oversees the operation of the power plant. The remote monitoring of the electrical signals at a distant location provides uncorrupted data. When the vibration levels of an individual power generator increased, the generator could be scheduled for maintenance repair during off-peak hours.
The reduced size and power consumption of the fiber optic sensor provides for ease of deployment in a low-profile package. The power consumption of a sensor-based system at a remote location, as described herein, will always be substantially lower than that of a corresponding laser transmission system since the power required to drive the laser is in addition to that required to drive the microwave or RF preamplifiers and is equal or greater in magnitude than that required to drive microwave amplifiers.
Applications of this sensor configuration extend to fiber optic Transmission Systems including Local and Wide Area networks LAN's and WAN's. The current configuration for fiber optic based networks is based on locating an optical source at each computer. This results either in high cost or low performance. High performance such as long distance between network nodes can presently be achieved using high power lasers at each node. The performance is achieved at the expense of providing a high power laser at each node. In order to lower installation costs, low power LED's or other sources have typically been used. The result is that in spite of the low loss of optical fibers, the system performance is very poor with low cost optical sources. In almost every case, the modulation applied to the optical beam is readily available to anyone able to access the fiber and thus not provide any measure of security.
Low cost electric field modulators incorporated into the coherence selective sensor system would provide reduced costs at each network node. A single high performance optical source at a file server would provide probe light necessary to interrogate the sensors located at each node. Due to the incoherence of the return signals from the modulators at each node, a relatively secure network is practical. The high power laser source can be divided down using optical splitters to address many nodes.